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Lecture 6 Chapter 5: Extensions and Modifications of Basic Principles Not negation of Mendel’s principals, but additions to them. Codominance: Phenotype of the heterozygote includes the phenotype of both homozygotes The ABO Blood Group: codominance, dominance, and multiple alleles Penetrance: Percentage of individuals with genotype that express the expected phenotype. Polydactyly: normally from a dominant allele, but not always. Cancer, Alzheimer’s Expressivity: The degree to which a trait is expressed. Polydatyly Cancer, Alzheimer’s Which alleles are codominant? Which alleles show dominance/recessiveness? 1 Segregation of lethal alleles can alter Mendelian ratios. So far today: all of these situations involve interactions of different alleles at single loci. Previously, dihybrid or trihybrid crosses have involved genes (loci) that independently assort, AND The genes are independent in their phenotypic expression: 9/16 3/16 3/16 1/16 R_Y_ R_yy rrYy rryy round, yellow round, green wrinkled, yellow wrinkled, green In gene interaction, genes at different loci contribute to determination of a single phenotypic characteristic. Gene interactions can lead to modified Mendelian ratios. What’s odd about this figure? The Y allele is recessive with respect to lethality! Gene Interaction for Fruit Color in Peppers Genotype R_C_ R_cc rr C_ rr cc Phenotype red brown yellow green Molecular Reason red pig., decomp. chlor red pig., chlor. persists no red, decomp. chlor. no red, chlor. persisits Gene Interaction with Epistasis Epistasis: the masking of the expression of one gene by another gene at a different locus. The gene that does the masking is the epistatic gene. 1. Recessive epistasis 2. Dominant epistasis 3. Duplicate recessive epistasis Hint: Ratios involving 1/16s suggest that two loci are interacting. A test cross doesn’t give the same frequencies. 2 Recessive epistasis in Labrador retrievers Black Chocolate Yeller B: black pigment b: brown pigment E: deposition of the pigment into hair shaft e: no deposition into shaft Genotype B_E_ bbE_ B_ee bbee Phenotype Black Brown Yellow Yellow P: BB EE x bb ee Black x yellow F1: BbEe Black e is epistatic to B or b BbEe x BbEe F2: B_E_ bbE_ B_ee bbee B: black pigment b: brown pigment E: deposition of the pigment into hair shaft e: no deposition into shaft 9/16 Black 3/16 Brown 3/16 Yellow 1/16 Yellow 9/16 Black, 3/16 Brown, 4/16 Yellow = recessive epistasis Dominant Epistasis Dominant Epistasis Genotype W_Y_ W_yy wwY_ wwyy Color in winter squash P: white squash x green squash F1: Phenotype white white yellow green white squash white squash x white squash F2: 12/16 with white squash 3/16 with yellow squash 1/16 with green squash How can gene interaction explain this? 3/4 produce white squash, 1/4 produce squash with color. A dominant allele at one locus can block color? W_ blocks pigment production. ww allows pigment. ¼ Then among plants with pigment: 3/4 and Thus a second locus: Yellow and Green Y_ yy 12/16 blocked by W_ 1/16 have ww to allow green AND yy that blocks production of yellow 3/16 have ww to allow green production AND Y_ to convert green compound to yellow. Many cases of epistasis: a gene having an effect on an early step in a biochemical pathway is epistatic to genes affecting later steps. 3 Duplicate recessive epistasis Albinism in a freshwater snail P Another way to look at this duplicate recessive epistasis aaBB AAbb albino x albino 7/16 albino F1 AaBb x AaBb F2 9/16 pigmented AaBb pigmented 9/16 A_B_ 3/16 aaB_ 3/16 A_bb 1/16 aabb pigmented albino albino albino 7/16 albino Look at A locus: ¾ are A_ And thus make Enzyme 1 Look at B locus ¾ are B_ And thus make enzyme II Both traits, when recessive, are epistatic: need presence of aa or of bb to block production of colored pigment. ¾ x ¾ Make both A enzyme and B enzyme = 9/16 Make pigmented compound C Epistatic genes can be dominant or recessive. One gene effects the other genes, but they are still assorting independently. Before lab next week: Read Chapter 6, pp 132-139. 4